The present invention generally relates to the field of communication systems and more particularly, to estimating the time and frequency response of at least one desired signal received by at least one antenna, based on Gradient Optimization techniques.
In a wireless communication system, a major design challenge is to maximize system capacity and performance in the presence of interference, and a time-varying multipath channel. Multipath propagation is caused by the transmitted signal reflecting off objects near the transmitter and receiver and arriving at the receiver over multiple paths. Interference in a communication system can come from a variety of sources depending on the particular system deployment. If the system is in motion, then Doppler-induced channel variations become an issue. Interference and multipath are major factors that limit the achievable performance and capacity of a communication system because both effects interfere with the ability of a communication receiver to properly decode the transmitted data.
In a multipath propagation channel, the transmitted signal propagates to the receiver over a finite number Lp of propagation paths, where each path has an associated time delay and complex gain. In such a channel, the communication receiver receives the superposition of Lp delayed, attenuated, and phase-shifted copies of the transmitted signal. The number of paths Lp and their time delays and phase shifts depends on the physical location of the various scattering objects (such as buildings, automobiles, and trees) in the immediate vicinity of the transmitter and receiver. The complex attenuation (magnitude and phase) of each path depends on the length of each path, as well as the material composition of any scatterers or reflectors encountered along the path.
The presence of multipath can severely distort the received signal. In a multipath environment, the multiple copies of the transmitted signal can interfere constructively in some portions of the occupied bandwidth. In other portions of the occupied bandwidth, the multiple copies can interfere destructively at the receiver. This signal duplication causes unwanted variations in the received signal strength over the bandwidth occupied by the signal. Furthermore, if the difference in the path delays of the various propagation paths is significantly greater than the duration of a transmitted information symbol, then intersymbol interference is present at the receiver. When intersymbol interference is present, the received signal is corrupted by prior transmitted symbols propagating over paths having delays relative to the shortest path that are longer than the duration of an information symbol. The demodulation process (the process of determining which information symbol was transmitted) becomes difficult in the presence of intersymbol interference.
In a mobile wireless communication system, the complex attenuation of each of the multipath components of the received signal becomes a time-varying function of the transmitter""s path and speed throughout the scattering field local to the transmitter""s position. The transmitter""s motion causes the received signal strength at a particular portion of the occupied bandwidth to vary as time progresses. In a mobile multipath channel, the overall channel response not only varies across the occupied bandwidth of the signal, but also across time as well.
In addition to multipath, interference is another system component that limits the performance of a communication system. If the system is deployed in an unlicensed band, then other users of the band can generate interference. And in a cellular system employing frequency reuse, transmitters in another cell that is allocated the same set of frequency channels can generate co-channel interference. Frequency reuse is the practice of assigning the same frequency channels to multiple users of the allocated spectrum.
Many cellular communication systems employ the technique of frequency reuse in order to maximize the utilization of the frequency spectrum allocated to a wide-area system deployment. In a cellular system, a large geographical area is divided into smaller regions called cells, where each cell is served by a single base station operating on an assigned set of frequency channels. Within each cell, multiple subscriber devices are allowed to communicate with the base station on the frequency channels assigned to that cell. The concept of frequency reuse involves allocating different sets of frequency channels to the cells belonging to a particular group and then reusing the same sets of frequencies to the cells belonging to another group of cells.
The reuse factor of a cellular system is defined to be the minimum distance between two cells that are allocated the same set of frequency channels divided by the radius of a cell. A cellular system employing a large reuse factor does not utilize the allocated spectrum as efficiently as a cellular system employing a smaller reuse factor. However, the level of co-channel interference received by a receiver in the cellular system is directly dependent on the reuse factor. Reducing the reuse factor tends to increase the level of co-channel interference experienced by a receiver. To better utilize the available spectrum, it would be advantageous to be able to suppress the effects of co-channel interference.
To compensate for co-channel interference, multi-user channel estimation is used. The current multi-user channel estimators require a very large matrix inversion which is based on each user""s pilot sequence or symbol decisions to be solved to find a matrix of filtering coefficients that, when multiplied by the received data, gives the channel estimates. This requires processing and storing a large matrix that can be burdensome to processor and memory limited implementations. Also, there is a need to perform a large matrix inversion based on each user""s pilot sequences or symbol decisions. The inverse is not a problem when the sequences are known since this inverse is computed once upon device initialization and then stored. However, the large inverse makes decision-directed channel updates impractical in real time.
Thus, there is a need for a method and device for estimating the time and frequency response of at least one transmitted signal received on at least one receive antenna. In addition, there is a need for a method and device to eliminate the need for computing and storing the inverse of a very large matrix.